Violent protests continued in Greece last night as hundreds of rioters fought running battles with police in central Athens and firebombed the offices of a credit checking agency.

The violence erupted following a memorial gathering at the spot where 15-year-old Alexandros Grigoropoulos died on 6 December after being shot by a policeman.

The rioters, using the National Technical University of Athens as a base, launched attacks against police, throwing rocks and petrol bombs and erecting roadblocks. Police responded with volleys of tear gas.

Greek law prevents security forces from entering the university grounds unless the school's administration gives the go-ahead. So far no permission has been given.

Grigoropoulos's shooting touched a nerve among Greek youths, who took to the streets to protest at what they see as random police violence.

The protests, which are now running into their third week, have been fanned by perceptions of corruption among politicians and poor job prospects as the economy takes a turn for the worse.

Yesterday's clashes dashed the hopes of the government and police that protest fatigue would set in, as Christmas neared.

During the evening an office block housing Tiresias SA, a credit checking agency, was targeted by arsonists and destroyed. Two cars were also torched.

Earlier about 150 youths defaced central Athens's Christmas tree, hanging bin liners from its branches, before clashing with riot police. The square was cleared within two hours. The tree, which was a replacement for the one burned down two weeks ago, survived the attack after riot police with shields formed a circle round it while protesters danced around them holding hands.

In Thessaloniki a group of self-styled anarchists briefly occupied a radio station and a cinema before disrupting an open-air charity event attended by the city's mayor.

The protests have caused hundreds of millions of euros damage, rocking a conservative government that has a one-seat majority and trails the opposition in the polls. Some analysts say continued street protests could force early elections. Unions and students have organised more protest rallies for the new year.

The policeman accused of shooting Alexandros Grigoropoulos has been charged with murder.

The Greek prime minister, Costas Karamanlis has rejected calls to step down, despite growing public pressure. But he has acknowledged that "long-unresolved problems, such as the lack of meritocracy, corruption in everyday life and a sense of social injustice" are fuelling the anger of young people.

Ripples from the Greek protests were felt in Germany, where about 1,000 people turned out for a demonstration in Hamburg. Four people were injured and nine arrested after bottles were thrown at police.

A further demonstration by about 250 people near the Greek consulate passed without incident, a police statement said.